Recently, with progress in the non-contact three-dimensional shape measurement technique, it has become easier to obtain a three-dimensional model of a natural object, where there lacks a design drawing or CAD data, such as a human body. As a result, three-dimensional models are finding applications in a number of technical fields. A three-dimensional shape model of a human body is used in a variety of sectors, such as apparel, medical treatment or computer graphics (CG). In these days, also such a technique has been proposed which utilizes the three-dimensional shape data of a human face for authentication of individuals.
As non-contact three-dimensional shape measurement technique, a variety of techniques, such as the flying time measurement method, moiré measurement method or the pattern projection method, have so far been proposed. In general, the flying time measurement method is a particular technique, both for transmission and reception, and is in need of a high precision measurement apparatus. The moiré measurement method allows for measurement only of a relative shape with respect to a reference shape. In contrast, the pattern projection method allows shape measurement to high precision at a relatively lower cost. In addition, the pattern projection method has various merits, such as allowing for photographing a textured image with the same camera as that used for shape measurement. Larger numbers of three-dimensional shape measurement apparatus, employing the pattern projection technique, have been fabricated and presented to the market. In a classical pattern projection method, a projector-camera set is used, and measurement is made of the three-dimensional shape (depth) based on the principle of triangulation. Initially, a light pattern, encoded with the direction of projection (position of an intersection of the light ray and a projector grating) as phase, is projected by the projector, and a pattern, obtained on reflection, is image-shot by the camera. A luminance pattern of the shot image is then decoded to restore the phase. Geometrical model parameters, such as the position or the orientation of the projector or the focal length of the projector lens, are determined from the outset, whereby it becomes possible to determine a line (or a plane) of projection directed to a target point for measurement by the projector based on the phase restored. In case geometrical model parameters of the camera are also determined from the outset, the line of sight of the camera, associated with a pixel of interest, may be determined. A three-dimensional coordinate of the target point of measurement may be determined by finding a point of intersection of the line of sight of the camera and the projection line (plane) of the projector.
Depending on the sorts of projected patterns, the following three techniques are used as the main techniques for measurement of the three-dimensional shape by the pattern projection method. Although a light wave pattern is taken as an example of the projected pattern, the pattern may also be that of a sound wave, or any other wave. Thus, the terms ‘projector’ and the ‘camera’ may also be a ‘pattern projection unit’ or a ‘pattern imaging unit’ associated with any particular wave used.
Among the techniques used in the pattern projection method, a spot or slit light scanning method, employing laser light, is most popular. With this technique, an image is taken (shot, i.e., scanned) as the direction of projection of the laser light is gradually changed along time, and the shot images are subjected to binary coding to determine a line (or plane) of projection from the projector for the detected point of observation. This system is robust against ambient light and allows measurement to remote points because of strong contrast obtained with the laser light. However, measurement may be made only of a single point by one-step projection and image-shooting. In the case of scanning by slit light, only a single point on a curve may be measured. Thus, to effect high density measurement in a short time, a special mechanism that allows high speed scanning and image-shooting at a high frame rate is required. This known technique may be deemed to be a pattern projection method with the use of a delta function as a projection pattern, with the phase being equivalent to the position of intersection of a projector grating and the projected laser light. For example, assume that the direction of illumination of the laser light is changed at a constant angular velocity ω and that the luminance value of a given pixel becomes higher than a threshold value at a time point τ. If, in this case, a projector grating is positioned at a distance equal to 1 from the projector center, the phase, which is the position of the point of intersection of the projected light with the projector grating, is as follows: φ=tan(ωτ).
The spatial coding method is such a technique that projects a binary pattern changing with time to encode the direction (plane) of projection so that each domain of the projector grating will be of a unique pattern. Liquid crystal projectors, capable of projecting variegated patterns, have now come into widespread use. Thus, the spatial coding method has become popular as being a technique that allows a range finder to be constructed inexpensively only with the use of general-purpose components without using special systems, such as laser scanners. With this known method, depth resolution equal to as high as 2″ may be obtained with an n-number of times of projections. Thus, the number of times of projection necessary to achieve the same depth resolution may be lesser than the case with the spot or slit scanning method. The n-bit digital value, restored with the present known method, corresponds to an encoded digital value of a phase which is the position of intersection of the projector grating with the projected light.
The phase shifting method is the technique of projecting a pattern obtained on encoding the direction of projection with the phase of the analog value. This known method allows a high depth resolution to be obtained with the number of projection steps further smaller than the case with the above techniques. This known method uses a sinusoidal pattern, obtained on encoding the direction of projection as an initial phase, and shoots images as the phase is changed for one period. The phase corresponding to an encoded direction of projection is calculated by fitting a sine wave to time changes of luminance values of the respective pixels of the shot image(s). The present known method allows high-speed three-dimensional shape measurement because the number of the projection patterns may be reduced to a number as small as three as the minimum value. The method also has a number of merits. Thus, as a principle, three-dimensional shape measurement may be made simultaneously for all pixels of the image, and a textured image may be measured at the same time as the image shape is measured. Further, depth measurement may be improved in accuracy by increasing the number of steps. In particular, measurement to high precision may be achieved even with a smaller number of steps by projecting a plurality of repeated patterns and by shortening the depth distance corresponding to one period.
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